1. Field of the Invention
The present invention relates to a method for freeing and releasing hollow tubular members stuck in a borehole. More specifically, the present invention relates to freeing a drill string which has become stuck to the side of a wellbore due to differential pressure between the wellbore and the adjacent formation.
2. Description of the Prior Art
In drilling wells, a common problem is sticking of the drill string in the well. Once stuck, substantial time and money are often lost in trying to free the drill string. Steps taken typically include circulating an oil-base fluid into the annulus around the stuck drill string and using drilling jars in the string. Too often the efforts prove unsuccessful and the drill pipe must be severed above the stuck point to remove the free portion from the well. The stuck portion left behind must then be removed by specialized "fishing" techniques and tools. If this proves uneconomic or impossible, the well must either be abandoned, or it may be sidetracked and redrilled below the point at which the drill pipe became stuck. None of these prior art techniques is satisfactory.
Especially troublesome, particularly in wells penetrating formations having lower than normal pore pressures, is sticking due to differential pressure between the formation and the wellhole. The pressure in the wellbore will preferably always be greater than the adjacent formation pressure, to prevent formation fluids from entering the wellbore. Greater than normal pressure differentials may be encountered, however, in partially depleted reservoirs where production of offset wells has reduced the original pressure of the zone. Normally, this pressure differential can be maintained by a low-permeability filter cake deposited on the walls of the wellbore. A filter cake is formed by the deposit of clays and polymers found within the drilling fluid, and the term is well known to those skilled in the art.
Generally, the part of the drilling string which becomes stuck is a drill collar. Drill collars are larger in diameter than the drill pipe and, therefore, are more likely to bind or jam in narrower or deviated portions of the wellbore. Their larger diameters also present a greater area for the collar to contact the wall of the wellbore. Where greater than normal differential pressures exist, this causes higher sticking forces directed from the wellbore to the formation. Experience shows that partially depleted reservoirs which have lower formation pressures are more likely to have a stuck drill string, particularly at the drill collars, than a less depleted formation have smaller pressure differentials.
A need thus remains for an economical, rapid, and effective method for freeing and releasing a hollow tubular member, such as a drill string, which has become stuck by differential pressure against the wall of a wellbore.